In laser based photo acoustic spectroscopy, each molecular specie in a detection chamber is basically detected from the response to illumination by laser radiation of a specific wavelength. Absorption of such radiation by a specie in the detector chamber at the specific wavelength produces an amplitude modulated pressure which is detected by a microphone in the detector chamber. Generally, if more than one specie is involved, with interference from unwanted species is to be taken into account, then operation at corresponding different wavelengths is required. The procedure involved ultimately sorts out different species and/or interfering components.
Such a procedure normally requires the operation of a spectrophone at different wavelengths in time sequence, that is to say requires that the laser be tuned in time ordered sequence to different wavelengths. When each wavelength arises from a different and separate source, such as a set of semi conductor lasers each operating at a different wavelength, the illumination from each such laser is injected into the spectrophone in timed sequence. Thus, measurement of multiple species cannot be carried out simultaneously and consequently requires more time for the species to be identified and their concentrations determined.
Further information in this respect can be found in the following references:    Kreuzer, L. B., Journal of Applied Physics, 42, 2934 (1971).    Rosengren, L-G., Infrared Physics, 13, 173 (1973).    Minguzzi, P., Tonelli, M., and Carrozzi, A., Journal of Optical Spectroscopy, 96, 294 (1982).    Morse, P. M., “Vibration and Sound” (McGraw-Hill, New York, 1968).    West, G. A., Barrett, J. J., and Siebert, D. R., Review of Scientific Instruments, 54, 797 (1983).
It is therefore an object of this invention to provide a method simultaneously identifying and determining the concentrations of multiple species by means of a spectrophone.